This invention relates to telecommunications systems, and more particularly to echo cancellation systems used in transmission, switching, and other components of telecommunications systems, including packet-based and Internet voice networks.
Echo remains a significant problem in voice and certain other telecommunications systems that incorporate physically lengthy transmission paths or other sources of significant transmission or propagation delay. Echo is typically caused when a signal generated by a xe2x80x9ctalkerxe2x80x9d and transmitted from a first end of a communications link is partially regenerated at a second end and returned to the first end of the link. By convention in the telecommunications arts, the end of the link containing the echo sourcexe2x80x94that is, the second endxe2x80x94is considered the xe2x80x9cnearxe2x80x9d end of the link, and the talker is located at the xe2x80x9cfarxe2x80x9d end of the link. The regenerated, or xe2x80x9cechoxe2x80x9d signal is received by the talker at the far end of the link, and can degrade the talker""s perception of naturally generated speech from the near end. Echo may occur even when the communications link is formed from two isolated unidirectional communications paths operating in opposite directions, because devices at the ends of the link (or elsewhere) may receive a signal on one path, and transmit a regenerated by-product on the other path.
Depending on the amplitude and delay of the echo signal and its similarity to the original signal, the echo signal may be more or less noticeable to a user. When the by-product or xe2x80x9cechoxe2x80x9d signal has significant amplitude, and is delayed by more than about 20-30 mS, the echo signal may be sufficiently distracting as to make conversation difficult.
Echo may be produced in a number of ways. In conventional telecommunications transmission plants, a typical producer of echo has been the xe2x80x9chybridsxe2x80x9d used as converters between four-wire transmission facilities and two-wire loops. Despite excellence in hybrid design, some leakage nonetheless occurs from the inbound path to the outbound path. When leakage occurs at a point xe2x80x9cclosexe2x80x9d to the talker, the echo signal typically arrives with so little delay that it is neither noticeable or disturbing. However, when leakage occurs at the near end, such as at the near-end hybrid, the arrival of the echo signal at the far end may be significantly delayed due to the physical length of the transmission path and certain other network components. In that case, the echo signal may be noticeable; in some cases, the echo signal is so disturbing that conversation is difficult.
Several other network devices may also cause delay, even where physical path lengths are relatively short. For example, in modern mobile or wireless telephone systems and Internet voice systems incorporate voice coding devices (sometimes known as xe2x80x9cvocodersxe2x80x9d), which may introduce a significant delay. Even as telecommunications providers convert the world""s telecommunications networks from analog to digital technologies, and despite the continual improvement in the performance of network components, some existing sources of echo remain, and new ones are created.
A variety of systems have been developed to minimize the effect of echo on the quality of communications service provided. When long-distance telecommunications systems were dominated by analog transmission facilities, the characteristics of individual transmission paths were carefully engineered to insert a controlled amount of attenuation. The attenuation was intended to reduce the amplitude of the echo signal such that it was not noticeable to the user. Although this system worked relatively well, it was primarily applicable to analog transmission facilities, and required significant continuing maintenance efforts and expense to adjust the attenuation levels to their engineered values.
Other systems have been developed to eliminate the effect of the echo signals without requiring careful control of the attenuation of transmission facilities. These systems are particularly necessary for digital telecommunications transmission systems, in which it is not feasible or desirable to introduce attenuation in the message content being carried by such transmission systems, but they have also been applied to analog transmission systems. These echo-control systems have incorporated two different main technologies which have been broadly characterized as xe2x80x9cecho suppressersxe2x80x9d and xe2x80x9cecho cancellers.xe2x80x9d
Echo suppressers employ one or more speech detectors, and one or more switches in the audio paths of a telecommunications link. For example, in one known echo suppresser, a speech detector monitors the near-end receive path and responsively controls a switch that enabled the near-end transmit path. If speech is not detected (i.e., if the near end is not talking), the echo suppresser disables the transmit path, thus preventing the locally generated echo signal from being transmitted to the far end. Echo suppressers of this type work well provided only one party talks at a time, but work poorly when the parties interrupt one another or talk simultaneously, as is characteristic of normal conversation.
Improved echo suppressers have incorporated speech detectors on both transmit and receive paths, and enable the transmit audio path responsive to a comparison of speech levels on the respective paths. When both parties talk simultaneously, the suppresser may leave the transmit path enabled, resulting in no echo suppression during that period, or may attenuate the transmit path to an intermediate level. Echo suppressers have not provided entirely satisfactory results, in part because some echo remains detectable during periods of simultaneous speech, and because the frequent switching of audio paths results in numerous abrupt changes in speech amplitude which are noticeable to the users.
Echo cancellers construct a model of the round-trip signal path through the network (e.g., the path from the echo canceller to the leakage source at the near end, and back to the echo canceller) that results in the echo signal. Using the model, and based on the original signal transmitted from the far end to the near end, the echo canceller calculates an estimated echo signal which it expects to receive from the near end. The echo canceller then subtracts the estimated echo signal from the received near-end signal. If the model is good, the estimated echo signal closely approximates the actual echo component of the received signal, and the echo is effectively subtracted away or canceled. Thus, substantially only the signal originally transmitted by the near end remains.
Although effective echo cancellers are available, they are expensive. Historically, echo cancellers have been permanently installed to serve particular telecommunications facilities (e.g., trunks). However, echo control is not always required or desirable on a facility at all times or for all calls. For example, facilities may not be continuously in use. Also, some calls, such as those carrying certain types of data may be hindered by the action of echo cancellers. Some existing echo cancellers can detect that a served facility is carrying a data call of a type for which echo cancellation is not desired and may responsively disable cancellation.
Even for calls which do not carry data, conditions at the near end and/or over the communications path may be such that echo control is unnecessary. For example, the near-end leakage signal may be of small amplitude, attenuation along the communications path may be significant, the length of the path may be short, or another echo cancellation device may be present on the call. Any of these conditions could produce an echo signal which either is not noticeable to the user or does not disturb communication. Because a facility served by an echo canceller may be used in conjunction with various other facilities and intermediate and near-end equipment from call to call, echo control may be essential on some calls and superfluous on others.
However, it is believed that no known echo cancellers detect facility conditions or other characteristics of a call that render echo control unnecessary and respond accordingly to deactivate the echo canceller. This is the case even when another echo cancellation device may be present on a call or circuit. A protocol is available in which a signaling parameter alerts other switches that an echo cancellation device is already present on a call or circuit, allowing those switches to avoid activating their own echo canceller. However, in practice, the signaling parameters have not been properly implemented by all equipment vendors and some calls that should receive echo cancellation do not. Accordingly, service providers have been ignoring the signaling parameters and always attaching an echo cancellation device. If no echo is present, it is assumed that the echo cancellation device will not significantly affect the circuit or call to which it is attached. However, this is an inefficient use of expensive resources and where echo cancellers are managed as a group, increases the holding times of the entire group.
Conventional echo-cancellers are provided via dedicated wiring arrangements or through digital cross-connect systems that do not allow per-call configuration. In traditional long-distance networks, this has not been a penalty, since transmission circuits could be identified as long distance or local, and only the fraction of trunks (typically 40%) used for long distance need echo cancellers. However, wireless and Internet gateway trunks are not identified as long distance, and therefore, it is impossible to predict whether a particular trunk requires echo cancellers. In addition, there are no other commonly-stated qualities of a trunk on which to base a decision as to whether to provide echo cancellers. Therefore, service providers are deploying echo cancellers on a wide scale.
Furthermore, for Internet voice calls can be connectionless, which means that traditional circuit rules for echo canceller engineering do not work, and traditional physical wiring and cross-connect points do not exist. When echo cancellers are equipped on a switching fabric, they have the appearance of a resource pool. The engineering parameters needed by equipment vendors and service providers to correctly provision echo cancellers equipped as resource pools on a switching fabric have not yet been fully developed.
Moreover, although there have been attempts at per-call control of echo cancellers across long-distance networks, this has not been successful, and in some networks, echo cancellers have had to be deployed on all CLEC trunks. As these interfaces increase, the percentage of trunks needing echo cancellers is exploding. In wireless and Internet equipment, echo cancellers are being provided as service circuits equipped on the switching fabric to reduce the cost of expensive wiring and cross-connect equipment and to cope with the virtual nature of Internet facilities.
Also, conventional echo cancellers may be arranged as individual, self-contained units, or as a plurality of echo canceller channels provided by common equipment. Deactivating the echo canceller associated with a facility carrying a call idles an expensive resource. It is believe that in existing echo canceller systems, once an echo canceller is assigned to service a facility based on an expectation that the facility will need echo control, there is no provision to automatically reallocate the echo canceller to another facility when it is determined that the first facility will not significantly benefit from echo cancellation.
It is therefore an object of the present invention to provide an improved echo canceling system which minimizes the aforementioned disadvantages of the prior art.
An echo canceling system with self-deactivation constructed according to the present invention comprises an adaptive echo canceller for applying echo cancellation to a communications signal, an echo comparison system for comparing the echo-canceled signal produced by the echo canceller with the untreated communications signal, and a switch for selecting as an output signal either the echo-canceled signal or the untreated signal. The comparison may be based on the respective energies of the two signals over a recent interval.
When the echo comparison system determines that the difference between the echo-canceled signal and the untreated signal is large, significant echo is present in the untreated signal, and the echo-canceled signal is used.
When the difference between the echo-canceled signal and the untreated signal is small, either little echo is present in the untreated signal, or the echo canceller is ineffective in removing the echo present. In either case, the echo canceller is not significantly contributing to the quality of the communications circuit. Accordingly, the echo canceller is deactivated and the untreated signal selected for use. The echo canceller may remain idle, or, preferably may be allocated to another facility.
According to another aspect of the invention, an echo canceling system designed to service N facilities may incorporate fewer than N echo cancellers (or aggregate echo canceling capacity for fewer than N facilities) where not all facilities will simultaneously require echo cancellation. When a communications session (e.g., a call) is initiated on a facility served by the system, the system initially allocates an echo canceller to that facility. If the system determines that insignificant echo is present, the echo canceller is deactivated, and preferably made available for allocation to another facility when needed.